is the completed picture in your inventory. if it is it should show up as a conversation option.

there is no need to wait by the gate. once you have done all you need to do you will be able to leave. you probably can't continue because you haven't shown the picture yet to those who need to see it. it is very important to show the picture to robert.

Open your inventory, look at the picture puzzle.Move one or 2 pieces in place until Samuel talks about the picture. I think the puzzle might not be completed and that is why the picture dialogue doesn't come up.